Standard thin film anti-reflective coatings on synthetic diamond optical elements have excellent performance in terms of minimising reflection, but are limited in high power optical systems due to the ease with which they are damaged. Due to high absorbance and/or poor thermal conductivity the anti-reflective coating tends to be the weak point in any synthetic diamond window resulting in a synthetic diamond window with a low laser induced damage threshold (LIDT). Furthermore, even if the absorption level of a thin film anti-reflective coating is relatively low, the thin film can still fail in high power density optical applications. For example, for a 20 kW laser system damage of thin film anti-reflective coatings is problematic and current thin film anti-reflective coating solutions are unlikely to be compatible with laser systems operating at 40 kW or more. Such high power laser systems are desirable for a number of applications including laser produced plasma (LPP) extreme ultraviolet (EUV) lithography systems to drive integrated circuit processing to smaller dimensions. Such extreme optical applications will require a synthetic diamond window capable of handling extreme power densities and this will require the combination of: (1) a synthetic diamond material with the required dimensions and desired bulk optical characteristics including low optical reflectance/absorption/scatter; and; (2) an anti-reflective surface finish capable of handling extreme power densities. Thin film anti-reflective coatings can also be problematic in terms of their mechanical integrity, e.g. if subjected to scratching or abrasion.
As an alternative to thin film anti-reflective coatings, it is known that anti-reflective surface patterns such as moth-eye structures can be formed directly in the surface of an optical window material in order to provide an anti-reflective surface finish without the requirement of a coating. While such anti-reflective surface patterns have been successfully fabricated in a range of optical window materials, the application of this technology to synthetic diamond windows has proved problematic. The anti-reflective performance of such surface finishes has been variable due to the difficulty in processing precisely defined surface patterns into diamond material because of the extreme hardness and low toughness of diamond material. Furthermore, the processing methods required to form anti-reflective surface structures in diamond material have resulted in significant surface and sub-surface crystal damage being incorporated into the diamond material. This surface and sub-surface damage in the synthetic diamond window causes a number of inter-related detrimental effects including: (1) a reduction in the laser induced damage threshold of the synthetic diamond window; (2) a reduction in the power at which the synthetic diamond window can operate; and (3) a reduction in the optical performance of the synthetic diamond window as a result of beam aberrations caused by the surface and sub-surface damage. As such it would be desirable to develop a process which forms precisely defined anti-reflective surface structure into a synthetic diamond window without introducing surface and sub-surface crystal damage so as to achieve a synthetic diamond window which has a low absorbance, a low reflectance, a high laser induced damage threshold, and high optical performance with minimal beam aberrations on transmission through the synthetic diamond window. In addition, it would be desirable to provide a process which is low cost, compatible with existing materials processing, and scalable over large areas.
In relation to the above, a number of prior art documents have disclosed techniques for fabricating anti-reflective surface structure into diamond window materials as discussed below. However, it is believed that none of the prior art techniques have achieved the combination of features as identified above.
In “Materials for Infrared Windows and Domes” [Daniel Harris, published by The International Society for Optical Engineering, 1999] it is disclosed at section 6.1.1 that a moth eye surface structure can be formed directly in diamond material to reduce reflection. Here it is disclosed that such a surface structure can be fabricated by first etching a reverse moth eye structure into silicon by lithographic techniques and then growing diamond material on the etched surface by chemical vapour deposition. The silicon is then dissolved to leave the diamond material with a moth eye structure. It is described that a multi-layer structure including an outer diamond layer with a flat outer surface has a reflectance of about 18% at a wavelength of 10 μm, the reflectance being dominated by single-surface reflectance from the front face of the outer diamond layer (15%). When the flat diamond outer surface is replaced by a moth eye structure, reflectance is reduced to 7% at a wavelength of 10 μm.
One problem with this approach is that the reflectance is still relatively high and this is due to the fact that precisely defined anti-reflective structures cannot easily be achieved in diamond material by the technique of etching a reverse moth eye structure into a substrate and then growing diamond material on the etched surface by chemical vapour deposition. Furthermore, growth of diamond material on patterned substrates can lead to an increase in crystal defects such as dislocations within the diamond material which adversely affect the optical properties of the diamond material. Yet a further weakness of this approach is that the final optical element will inevitably include early stage nucleation diamond which has reduced thermal conductance and increased optical absorbance.
U.S. Pat. No. 5,334,342 discloses a similar method of fabricating moth-eye surface structures in diamond material by patterning a reverse moth eye structure into a substrate, growing diamond material on the patterned substrate, and then removing the substrate to leave the diamond material with a moth eye surface structure.
J. F. DeNatale et al [Fabrication and characterization of diamond moth eye antireflective surfaces on Germanium, J. Appl. Phys. 71, 1388 (1992)] have disclosed a similar approach by patterning a germanium substrate with a surface relief (moth eye) structure and then over-growing a thin diamond film on the patterned substrate such that the thin diamond film retains the underlying surface structure of the patterned substrate. It is described that the progressive gradation in the effective refractive index between air and the composite substrate has reduced Fresnel reflection losses to below 1%. This provides a means of overcoming the high refractive index and surface roughness considerations that often limit optical applications of polycrystalline diamond thin films. However, there is no disclosure of how to fabricate such moth-eye structures in free-standing diamond windows and although reflection losses have been reduced to below 1%, there is no disclosure of the laser induced damage threshold of the diamond material which will be sensitive to the quality of the diamond material. The quality of the diamond material in this instance will likely be poor as it is grown on a patterned germanium substrate.
T. V. Kononenko [Formation of antireflective surface structures on diamond films by laser patterning, Applied Physics A, January 1999, Volume 68, Issue 1, pp 99-102] discloses an alternative to the substrate patterning and diamond over-growth technique disclosed in the previously described prior art. This paper describes diamond surface microstructuring by a laser ablation technique. The optical transmission of the diamond films was found to increase from 70% to 80% at a wavelength of 10.6 μm by forming a microstructured surface by laser ablation.
Douglas Hobbs [“Study of the Environmental and Optical Durability of AR Microstructures in Sapphire, ALON, and Diamond”, www.telaztec.com] has also reported the fabrication of moth eye anti-reflective surface microstructures directly in diamond material. It is reported that diamond windows with anti-reflective surface structures have been fabricated which have a transmittance of approximately 80% at a wavelength of 10 μm which compares with a value of approximately 70% for an untreated diamond window. These results appear similar to those reported by Kononenko using a laser ablation technique for patterning a diamond surface.
Hobbs also discloses that the anti-reflective microstructured diamond windows were tested for laser induced damage threshold using a pulsed CO2 laser operating at 9.56 μm with a 100 ns pulse length and a pulse repetition frequency of 4 Hz. It is indicated that results of the tests were variable and inconsistent due to the nature of the diamond material but that the damage thresholds measured were in a range 50 to 100 J/cm2, a level much higher than can be achieved with thin-film anti-reflective coatings.
Two key points may be noted from the Hobbs paper. First, the transmittance value of 80% is still rather low and this would indicate that the quality of the diamond material is relatively poor, the surface structures fabricated in the diamond windows are not precisely defined, or that significant surface or sub-surface damage has been introduced into the diamond crystal structure when forming anti-reflective surface micro-structures. Secondly, the paper does not indicate how the anti-reflective surface structures were fabricated in the diamond windows.
Previously described methods of fabricating anti-reflective surface structures in diamond windows have involved either substrate patterning and diamond overgrowth or direct patterning via laser ablation. An alternative technique is to directly etch anti-reflective surface structures into diamond windows. For example, various publications from Uppsala University in Sweden have focused on inductively coupled plasma etching of surface structures in diamond material including: M. Karlsson, K. Hjort, and F. Nikolajeff, “Transfer of continuous-relief diffractive structures into diamond by use of inductively coupled plasma dry etching”, Optics Letters 26, 1752-1754 (2001); M. Karlsson, and F. Nikolajeff, “Fabrication and evaluation of a diamond diffractive fan-out element for high power lasers,” Opt. Express 11, 191-198 (2003); and M. Karlsson, and F. Nikolajeff, “Diamond micro-optics: Microlenses and antireflection structured surfaces for the infrared spectral region,” Opt. Express 11, 502-507 (2003).
The Uppsala group have indicated that diamond-based optics provide an attractive alternative for high-power laser optics due to their damage resistance, reduced thermal lensing, and transparency from the UV to the far-IR spectral regions. The Uppsala group have highlighted the need for better surface patterning for diamond-based optics and have proposed an inductively coupled plasma etching approach which involves patterning a resist layer on an optical-quality synthetic diamond using direct-write electron-beam lithography followed by dry etching in an inductively coupled plasma (ICP). The gases used for the diamond etching are O2 and Ar and a typical ICP etch recipe is disclosed as comprising: gas flows of 7 sccm (standard cubic centimeters per minute) of O2, and 8 sccm of Ar; a chamber pressure of 2.5 mTorr; an ICP power of 600 W; bias voltages varied between −100 and −180 V; and sample etch times of between 2 and 20 minutes.
It has been indicated that by correctly designing and fabricating sub-wavelength anti-reflective structures on both sides of a diamond window, it is possible to increase the transmission at a wavelength of 10.6 μm from 71% (unstructured diamond) to almost 97% (for microstructured diamond). It is indicated that this improvement in transmission is very important for high-power lasers, in which even a fraction of the scattered high optical power can lead to severe problems. Applications of this technology are described as including outcoupling windows for neodymium-doped yttrium aluminum garnet (Nd:YAG) or CO2 lasers, satellite windows, and in x-ray optics. It is indicated that in these applications, it is mainly the high thermal conductivity, the high laser damage threshold, and the high wear resistance of the optical windows that are the driving factors.
Despite the above progress in fabricating anti-reflective surface structures into diamond windows, there is still a need to provide improved anti-reflective surface structures. It would be desirable to develop a process which forms precisely defined anti-reflective surface structure into a synthetic diamond window without introducing surface and sub-surface crystal damage so as to achieve a synthetic diamond window which has a low reflectance, a high laser induced damage threshold, and high optical performance with minimal beam aberrations on transmission through the synthetic diamond window. In this regard, while a number of prior art documents have disclosed techniques for fabricating anti-reflective surface structure into diamond window material as previously discussed, it is believed that none of the prior art techniques have achieved this desired combination of features. Furthermore, it has also been noted that a direct-write electron-beam lithography process for patterning of the resist prior to etching is time consuming and expensive.
In light of the above, it is an aim of embodiments of the present invention to provide a synthetic diamond optical element comprising an anti-reflective surface pattern formed directly in the surface of the synthetic diamond material and which has low absorbance and low reflectance while also having low surface and sub-surface crystal damage thus exhibiting a high laser induced damage threshold. It is a further aim to develop a technique for fabricating such anti-reflective surface patterns in diamond material which is relatively quick and low cost.